Nickel base superalloys have been extensively developed for use in gas turbine engines, mainly because of their superior high temperature mechanical properties. As engineers have increased the operating temperatures of these engines to increase the power output and efficiency, the need for materials which will withstand the higher temperatures has been of prime concern.
Significant improvements in alloy chemistry, processing and microstructure have been made, in order to meet these increased requirements. The development of columnar grain and single crystal materials has contributed even further to the elevated temperature capability of the materials.
The usefulness of columnar grain and single crystal materials is based on the principle that the greatest strength of the material is associated with particular crystallographic orientations. The successful use of these materials depends on aligning the preferred crystallographic orientation of the material to the maximum stress axis of the component being made. While the presence of grain boundaries in the columnar grain articles is less than ideal, the orientation of the grains with the (001) direction parallel to the grain boundaries takes advantage of the superior strength and creep resistance in this orientation. The absence of transverse grain boundaries avoids the detriment associated with having the weak grain boundaries aligned in the plane of highest stress, i.e., perpendicular to the loading direction. In this manner the optimum use of the material capabilities is made.
There are many applications where the unique properties of single crystal and columnar grain alloys are beneficial. A very useful application has been found in gas turbine engine components, where the anisotropic materials have permitted significant increases in operating temperature and efficiency. For example, both blades and vanes in the turbine section utilize anisotropic single crystal and columnar grain alloys.
The microstructure of PWA 1480, a nickel base single crystal superalloy material having a nominal composition of 10 Cr, 5 Co, 4 W, 1.5 Ti, 12 Ta, 5 Al, balance nickel, with all quantities in weight percent, consists of generally uniformly cubic shaped .gamma.' precipitate particles (approximately 60% by volume) in a continuous .gamma. matrix. The precipitate particles range in size from about 0.35.mu. to about 0.50.mu. and form an ordered array of face centered cubic (FCC) particles based on Ni.sub.3 Al(Ti). Cube edges of the precipitate particles are aligned with the &lt;001&gt; family of crystallographic directions. This microstructure is common to .gamma.' strengthened nickel base columnar grain and single crystal turbine component alloys.
The high temperature (1600.degree. F., or 870.degree. C., and above) property requirements are dictated by the envirornment in which the airfoil portion of a gas turbine blade must operate. Historically the primary consideration in turbine blade alloy development has been to achieve high temperature strength, creep capability, and oxidation and erosion resistance.
Significant advancements have been made in meeting these needs by the development of optimum alloy chemistry, microstructure and casting form (i.e., equiaxed, columnar grain, single crystal). Microstructural parameters for .gamma.' particle size, shape, volume fraction and .gamma.-.gamma.' misfit have evolved to where they are today to achieve an optimum balance between high temperature strength and resistance to creep. The optimization of the universally employed fine, uniform cuboidal .gamma.' precipitate particle structure has been fundamental in meeting this end.
U.S. Pat. Nos. 4,402,772, 4,643,782, 4,677,035, 4,802,934, 4,885,216, 5,077,141, 5,100,484 and 5,154,884 all disclose the formation of very small .gamma.' particles in order to obtain the optimum combination of mechanical properties in single crystal superalloy materials. The small particle size is obtained in prior art processing by cooling the material from the solutionizing temperature at a rapid rate, generally at least 100.degree. F. (56.degree. C.) per minute, followed by aging.
The attachment area, or root, of a turbine blade operates at a lower temperature than the airfoil portion, and presents a different set of operational requirements. Current trends in gas turbine technology emphasize damage tolerance (i.e., resistance to crack propagation) for the attachment areas. Microstructural parameters developed for high temperature capability required in the airfoils are not necessarily optimum for the crack growth resistance required in the attachment area.
We have investigated the micromechanics of fracture in the .gamma.' strengthened anisotropic superalloys and have observed fatigue crack growth to be highly dependent on .gamma.' precipitate morphology.
We have observed that, from about 800.degree. F. (427.degree. C.), the minimum stress intensity necessary to propagate a crack increases with increasing temperature up to about 1600.degree. F. (871.degree. C.). This is consistent with the tendency for .gamma.' strengthened superalloys to exhibit an increase in the critical resolved shear stress (CRSS), and consequently yield strength, with increasing temperature. This behavior demonstrates that .gamma.' deformation mechanisms affect fracture.
The microscopic failure mode under these conditions indicates that the prior art precipitate morphology responds to fatigue crack growth as a homogeneous isotropic solid, resulting in a singular microscopic fracture mode. This mode is characterized as trans-precipitate non-crystallographic fracture. Our experience has shown that a mixed mode fracture propagates more slowly.
Damage tolerance also becomes a critical requirement when anisotropic .gamma.' strengthened superalloys are used in hydrogen fueled rocket engines. The critical operating condition for a rocket turbine component is where maximum stresses occur at relatively low temperature in a high pressure gaseous hydrogen environment.
Post-test fractographic analysis of failed specimens reveals differences in the operative microscopic fatigue crack growth fracture modes observed in air and hydrogen. Fractures produced in air exhibit crystallographic crack propagation predominantly on microscopic (111) octahedral planes. Fractures produced in hydrogen exhibit preferential fatigue crack propagation in the .gamma. phase region of the microstructure in the vicinity of the .gamma.-.gamma.' interface, essentially parallel to the (001) crystallographic planes. Fatigue crack growth under these conditions is greatly accelerated. FIG. 2 illustrates the effect of 5000 psi (34.5 MPa) hydrogen on the fatigue crack growth rate of PWA 1480 having the prior art microstructure (ordered array of fine, cuboidal .gamma.' particles). The crack growth rate is approximately ten to 100 times greater in hydrogen than in air.
Columnar grain and single crystal materials as applied to gas turbine engines use are similar in nature in that the crystallographic orientation of the material is essentially the same. Even though the columnar grain materials have grain boundaries, which are generally associated with a weaker material, the orientation of the grain boundaries is generally parallel within about 15.degree. of the direction of applied load, and the grain boundaries consequently do not present an interface which is significantly stressed by centrifugal loading. Thus the key microstructural features that affect hydrogen fatigue crack propagation are common to both single crystal and columnar alloys, i.e., a geometrically ordered cuboidal .gamma.' precipitate array with cube edges oriented coincident with the (001) crystallographic directions. This configuration positions planar fields of the weaker .gamma. phase coincident with the plane of crack propagation in an (001) loaded system.
What is needed are single crystal and columnar grain materials having improved resistance to crack growth (damage tolerance) in hydrogen fueled or conventional gas turbine engines.
What is also needed is a process for creating the damage tolerant single crystal and columnar grain materials.